See: Synchronized Multimedia Integration Language (SMIL) 1.0 Specification

Please understand that I'm not promoting SMIL above any and all theoretical alternatives. It's simply an existing, current standard that appears to fit the bill of being a "standardized slideshow definition file" to some degree. If someone goes on to define another, better XML standard for slideshow interchange, I'd just hope they'd look at SMIL first to learn from, and not recreate, what exists.

Side note: I clearly haven't been keeping up with SMIL development. SMIL does have transitions and has had them for some time.